protected override void Update(GameTime gameTime)
{
    totalElapsedTime += (float)gameTime.ElapsedGameTime.TotalSeconds;

    // determine how far along the duration value we are (0 to 1), duration=5 sec
    float fraction = totalElapsedTime / duration;

   // fraction = MathHelper.Clamp(fraction, 0, 1);
    while (fraction > 1) fraction -= 1; 

    // interpolate modelPosition between startPosition and endPosition using fraction
    modelPosition = Vector3.Lerp(startPosition, endPosition, fraction);

    myModel.World = Matrix.CreateTranslation(modelPosition);

   /// 중간생략 …
   base.Update(gameTime);
}